1. Field of the Invention
This invention relates to an air-fuel ratio control system for an internal combustion engine, and more particularly to an air-fuel ratio control system for an internal combustion engine adapted to control the air-fuel ratio of an air-fuel mixture supplied to the engine based on output signals from air-fuel ratio sensors arranged in an exhaust passage at respective locations upstream and downstream of a catalytic converter, the output signals being inverted between a richer side and a leaner side with respect to a reference value when the air-fuel ratio of the mixture changes across a stoichiometric air-fuel ratio.
2. Prior Art
Conventionally, an air-fuel ratio control system for an internal combustion engine has been proposed by Japanese Provisional Patent Publication (Kokai) No. 58-48756, which includes, in addition to an air-fuel ratio sensor (O.sub.2 sensor) arranged in an exhaust passage at a location upstream of a catalytic converter, an O.sub.2 sensor arranged in the exhaust passage at a location downstream of the catalytic converter to compensate for an undesired variation among individual O.sub.2 sensors to be arranged upstream of the catalytic converter with respect to the output characteristic thereof, to thereby more accurately feedback-control the air-fuel ratio of the mixture based on an output signal from the upstream O.sub.2 sensor as well as an output signal from the downstream O.sub.2 sensor.
The downstream O.sub.2 sensor provided in the proposed air-fuel ratio control system detects the air-fuel ratio of the mixture supplied to the engine with a delay caused by oxygen storage effect of the catalytic converter. Therefore, if the repetition period of the inversion of the output signal from the downstream O.sub.2 sensor is long, an actual value of the air-fuel ratio of the mixture supplied to the engine can be largely deviated from the stoichiometric air-fuel ratio toward the richer side or leaner side when the signal delivered from the downstream O.sub.2 sensor is inverted with respect to a reference value or drastically changes. Although the air-fuel ratio of the mixture is controlled on average to the stoichiometric air-fuel ratio in such cases, the actual value of the air-fuel ratio of the mixture supplied to the engine undergoes an alternate large variation toward the richer side and the leaner side.
FIGS. 1a to 1d show an example of changes in the output signal FVO2 delivered from the upstream O.sub.2 sensor, the output signal RVO2 delivered from the downstream O.sub.2 sensor, and an air-fuel ratio correction coefficient KO2, when the repetition period of inversion of the signal delivered from the downstream O.sub.2 sensor is relatively large.
As shown in FIG. 1a, the output voltage FVO2 from the upstream O.sub.2 sensor undergoes prompt and frequent inversion between the richer side and the leaner side in response to changes in the air-fuel ratio of the mixture supplied to the engine since this sensor is free from the influence of oxygen storage effect of the catalytic converter. In contrast, the output RVO2 from the downstream O.sub.2 sensor is inverted at long time intervals. That is, with this sensor, relatively-long durations of the rich side output and the learner side output alternately occur.
More specifically, as shown in FIG. 1c, immediately after the output signal RVO2 from the downstream O.sub.2 sensor 17 has been inverted from the leaner side to the richer side, an enriching feedback control parameter PR assumes a maximum value and a leaning feedback control parameter PL assumes a minimum value, as shown in FIG. 1c, so that the air-fuel ratio correction coefficient KO2, which is determined by the alternate use of these P terms, continues to increase for some time in spite of respective stepwise decrease and increase of the feedback control parameters after the inversion of the output signal. On the other hand, immediately after the output signal RVO.sub.2 from the downstream O.sub.2 sensor 17 has been inverted from the richer side to the leaner side, the enriching feedback control parameter PR assumes a minimum value and the leaning feedback control parameter PL assumes a maximum value, as shown in FIG. 1c, so that the air-fuel ratio correction coefficient KO2 continues to decrease for some time in spite of respective stepwise decrease and increase of the feedback control parameters after the inversion of the output signal. As shown in FIG. 1d, after the enriching feedback control parameter PR is decreased to a minimum value m and the leaning feedback control parameter PL is increased to a maximum value n, at a time point corresponding to inversion of the output signal RVO2 from the richer side to the leaner side, the enriching feedback control parameter PR and the leaning feedback control parameter PL start to be increased and decreased, respectively. Then, when a maximum value o and a minimum value p are reached at a time point corresponding to inversion of the output signal RVO2 from the leaner side to the richer side, the enriching feedback control parameter PR and the leaning feedback control parameter PL start to be decreased and increased, respectively. Thus, the air-fuel ratio correction coefficient KO2 is controlled such that a waveform indicative of changes in the air fuel ratio correction coefficient KO2 has an envelope convergent to a desired air-fuel ratio (stoichiometric air-fuel ratio). However, since the correction coefficient KO2 is varied depending on the maximum value and the minimum value or the maximum value or the minimum value respectively assumed by the enriching feedback control parameter PR and the leaning feedback control parameter PL when the output signal RVO2 from the downstream O.sub.2 sensor 17 is inverted, so that the output signal RVO2 from the downstream O.sub.2 sensor 17 as a result of the feedback control by the use of the correction coefficient KO2 alternately continues to be on the richer side and the leaner side with relatively long durations. Particularly, when the output voltage RVO2 of the downstream O.sub.2 sensor is inverted after a long duration of the output signal on the richer or leaner side, the air-fuel ratio correction coefficient KO2 can be largely deviated from the desired value, resulting in degraded exhaust emission characteristics, i.e. emission of noxious components, such as CO, HC, and NOx, and degraded performance of the engine.
To solve problems in the air-fuel ratio control described above, air-fuel ratio control systems have been proposed e.g. by Japanese Provisional Patent Publication (Kokai) No. 63-120835 (hereinafter referred to "the first prior art") and Japanese Provisional Patent Publication (Kokai) No. 63-195350 (hereinafner referred to as "the second prior art"), the former being adapted to inhibit the air-fuel ratio feedback control based on the downstream O.sub.2 sensor, and the latter being adapted to increase a gain of an integral factor (I term) of the proportional-integral control (PI control) responsive to the output signal from the downstream O.sub.2 sensor, when the repetition period of inversion of the output signal is long.
In the first prior art, however, when the repetition period of inversion of the output signal from the downstream O.sub.2 sensor is long, the air-fuel ratio feedback control based on the output signal from the downstream O.sub.2 sensor is merely inhibited, and hence there is no difference from the air-fuel ratio feedback control by the use of only one O.sub.2 sensor arranged upstream of the catalytic converter. Therefore, it is impossible to compensate for variation in output characteristics of individual upstream O.sub.2 sensors when the repetition period of the output signal from the downstream O.sub.2 sensor is long, setting a limit to accuracy of the air-fuel ratio feedback control.
In the second prior art, the gain of the integral factor applied to the PI control for calculation of the feedback control parameters used in the air-fuel ratio feedback control is increased when the repetition period of inversion of the output signal from the downstream O.sub.2 sensor is long. However, it merely increases a rate of change in the integral factor per unit time. Therefore, it is difficult for the second prior art to promptly control the air-fuel ratio of the mixture supplied to the engine to the desired air-fuel ratio.